The increased variety and sophistication of detectors employed in conjunction with military weapons systems has served to highlight the critical need to better protect potential targets, both ground and airborne, from acquisition. This need is especially acute with respect to the radar frequency band of the electromagnetic spectrum as potential targets operating in the field are vulnerable to acquisition by a large plurality of radar systems operating over a broad band of frequencies, typically ranging from 2 to 200 GHZ.
Under ideal conditions a target can be provided, either during initial manufacture or through post-manufacture retrofit, with an absorber optimized for a particular radar frequency. It is, however, quite another matter to provide an absorber which is useful over a broadband frequency range encompassing the radar frequencies most likely to be encountered. While such broadband absorbers have hitherto been produced, they are typically quite bulky, heavy, and generally ill-suited for operational use in those situations where low bulk and light weight are of paramount importance. Furthermore, even with absorbers satisfying both low bulk and low weight requirements, it often turns out that the absorber construction is so complex as to render impractical its production in an economical manner.
Anechoic chamber technology has resulted in several innovative absorber structures exhibiting both low bulk and low weight. In this regard, U.S. Pat. Nos. 2,977,591; 3,568,196; and 4,012,738 each disclose absorber structures comprising a fibrous mat of non-conducting material having a layer incorporating conductive material therein which is deposited on the mat fibers and extends inwardly into the mat volume from one side thereof. In each patent the aforementioned layer is formed by suspending electrically conductive particles in a liquid binder selected, in part, for its ability to adhere to the mat fibers upon curing. The mat is then either sprayed with or dipped into the liquid mixture in such a manner as to produce a completed structure which contains an electrically conductive layer bound to the mat fibers and consisting of the thusly cured binder and electrically conductive particles. It should be noted that the layer so formed does not also fill the interstitial voids between adjacent fibers, thus maintaining mat porosity, and is characterized by a reduction in its thickness with inward progression from one side of the mat.
The drawbacks associated with such absorbers of the prior art are manifestly clear and include, in addition to the tedious nature of the methods for producing same, the fact that the electrically conductive layer is discontinuous in the sense that it consists of discrete electrically conductive particles bound to the mat fibers by the cured binder. This feature, coupled with the presence of the binder in the layer, detracts from the desired electrical properties of the absorber which would otherwise be obtained if the layer was formed of non-discrete material and free of the binder.
Accordingly, it would be highly desirable and beneficial, and there still exists the need, to provide an absorber which is characterized by having an electrically conductive layer which is substantially free of any materials therein detracting from the electrical properties of the layer, and which comprises a non-discrete electrically conductive layer as opposed to the layers of the prior art comprised of discrete particles. It would also be highly desirable and beneficial to provide an absorber, and a method for producing same, wherein control of the layer thickness with inward progression into the mat structure is more easily facilitated.